The sheet materials to which this invention is directed are usually referred to in the art as grain oriented "electrical" silicon steels or, more properly, silicon-irons. These silicon-irons are ordinarily composed principally of iron alloyed with from about 2.2 to about 4.5% silicon, small amounts of carbon, and various elements added to develop desirable magnetic properties. These sheet materials are of the "cube-on-edge" type, i.e., more than about 70% of their crystal structure are oriented in the (110) [001] texture, as described in Miller Indices terms.
Such grain oriented silicon-iron sheet products are currently made commercially by a sequence which typically begins by casting an ingot and hot rolling the ingot to form a strip or a sheet-like configuration, commonly referred to as "hot-rolled band", less than about 0.150" in thickness. The hot-rolled band is then subjected to one or more cold rolling operations, with intermediate annealing when two or more cold rolling operations are employed, to effect at least a 50% reduction in thickness. The cold rolled sheet is then heat treated for decarburization and subsequently heat treated again for development of the cube-on-edge secondary recrystallization texture.
Herbert E. Grenoble, in U.S. Pat. No. 3,905,842, herein incorporated by reference, teaches that alloying silicon-iron with a small amount of boron, in critical proportion to the nitrogen content of the metal, enables preparation of oriented silicon-iron of good magnetic properties from material which would otherwise be incapable of the secondary recrystallization necessary to develop such properties.
Another related disclosure concerning the use of small, but critical, amounts of boron in the production of silicon-iron with consistently good magnetic properties is that of U.S. Pat. No. 3,957,546 to Howard C. Fiedler, herein incorporated by reference, which defines a process for cold rolling hot band directly to the final gauge while maintaining the boron-to-nitrogen and manganese-to-sulfur ratios in the metal within certain critical ranges.
In U.S. Pat. No. 4,010,050 to Edward G. Choby, Jr., herein incorporated by reference, there is disclosed a nitrogen-bearing base coating useful in providing a more thorough distribution of nitrogen during the annealing of aluminum nitride inhibited silicon steels thereby overcoming difficulties experienced theretofore with nitrogen-bearing annealing atmospheres.
In U.S. Pat. No. 4,186,038 to Carl M. Maucione, herein incorporated by reference, it is disclosed that the addition of a very small amount of boron to an adherent electrically insulating coating during the final anneal of silicon-iron results in the development of substantially better magnetic properties than would otherwise be produced and can cause secondary recrystallization to take place when otherwise it would not. It is taught that the thus-coated steel may be final annealed in hydrogen, or a mixture of nitrogen and hydrogen, by heating to a temperature sufficient to cause secondary recrystallization and thereafter, if desired, in the same atmosphere, the heating may be carried on up to a higher temperature to effect complete removal of residual carbon, sulfur, and nitrogen.
More recently, Howard C. Fiedler, in U.S. Pat. No. 4,173,502, herein incorporated by reference, teaches that silicon-iron having substantially improved magnetic properties can be obtained consistently by limiting the amount of boron in the cold rolled and decarburized intermediate starting sheet material to one particular range and by limiting the amount of boron in an electrically insulating coating on the sheet to another particular range. It is further disclosed that it is essential to the consistent development of the improved magnetic properties that the total boron in the alloy and the coating be limited to a certain maximum and that the nitrogen content of the alloy be proportioned in a particular manner to the total boron content of the alloy and the coating.
Boron-containing electrically insulating coatings, as referred to above, may be provided by slurry coating or by electrolytic processes disclosed, for example, in U.S. Pat. Nos. 3,054,732 and 4,116,730 to McQuade and Arendt et al., respectively; the entirety of which are herein incorporated by reference. McQuade teaches the electrolytic application of a uniform coating of magnesium hydroxide (Mg(OH).sub.2) about 0.5 mil thick to the sheet. Thereafter, the sheet coated by the McQuade process is dipped in an aqueous solution of boric acid or sodium borate or other suitable boron compound solution, which is preferably relatively dilute, i.e., containing of the order of 5 to 10 grams per liter of the boron compound, to incorporate the boron. Alternatively, as taught in Arendt et al., the boron-containing magnesium hydroxide coating may be applied electrolytically in one step. Arendt et al. further broadly teach that the final annealing of silicon iron coated by their method may be conducted by heating in hydrogen or a mixture of hydrogen and nitrogen to a temperature sufficient to cause secondary recrystallization and that thereafter the heating may be carried on up to a higher temperature in the same atmosphere, if desired, to insure complete removal of residual carbon, sulfur, and nitrogen.